With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
In a 3G application for a system operating in the wideband code division multiple access (WCDMA) communication system, a portable transceiver operates in full-duplex mode. That is, both a receiver and a transmitter are operating simultaneously. In this operational mode, energy from the transmitted signal generated in the handset “leaks” into the receiver channel and generates a second-order inter-modulation signal component, which unfortunately falls in the same range of frequencies as the received signal.
Conventional approaches to reduce the transmitter signal leakage into the receive channel include the introduction of one or more surface-acoustic wave (SAW) filters. The transduction from electric energy to mechanical energy (in the form of SAWs) is accomplished by the use of piezoelectric materials. Electronic devices employing the SAW normally utilize one or more interdigital transducers (IDTs) to convert an acoustic wave to an electrical signal and vice versa using the piezoelectric effect of certain materials (e.g., quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, etc). These devices are fabricated utilizing photolithography, the process used in the manufacture of silicon integrated circuits.
SAW filters have been successfully applied in many cellular telephone architectures and provide significant advantages in performance, cost, and size over other filter technologies (e.g., digital signal processors, quartz crystals (bulk wave), LC filters, and waveguide filters). The continued drive in the industry toward reducing cost and device size, as well as the desire to realize increased efficiencies, makes it desirable to remove SAW filters from the telephone. However, removal of a SAW filter before a duplexer that couples both a receive channel and a transmit channel to a common antenna reintroduces the above-described interference in a desired receive signal due to second-order inter-modulation signal components from the transmit signal.
FIG. 1 is a schematic diagram illustrating the introduction of transmitter generated second order inter-modulation (IM2) in a desired receive signal of a conventional full duplex transceiver. The transceiver includes a transmit channel upconverter or TX upconverter 10, power amplifier 20, duplexer 30, and antenna 40 in a transmit channel. The transceiver also includes the antenna 40, duplexer 30, a low-noise amplifier 50 and a receive channel downconverter or RX downconverter 60 in a receive channel. A TX baseband signal containing information to be transmitted by the transceiver is upconverted from a baseband frequency to a RF frequency by the TX upconverter 10 before being amplified by the power amplifier 20. The frequency modified TX baseband signal is a RF transmit signal. The RF transmit signal, labeled TX signal and illustrated with arrows pointing toward the antenna 40, is amplified by the power amplifier 20 and coupled to the antenna 40 via the duplexer 30. A remotely generated receive signal, labeled RX signal and illustrated with arrows pointing to the right side of the figure, is received by the antenna 40 and coupled via the duplexer 30 to the low-noise amplifier 50. The low-noise amplifier 50 amplifies the RX signal and forwards the amplified RX signal to RX down converter 60 and other components in the handset for further baseband processing. When the portable transceiver operates in a full duplex mode, the duplexer 30 is simultaneously processing the RF transmit signal and the RF receive signal. The duplexer 30 is an imperfect device and provides limited isolation in the absence of a RX bandpass filter (e.g., a SAW filter), hence some portion of the transmit signal energy, labeled TX leakage, is coupled into the receive path of the transceiver.
The plot in FIG. 1 shows the relative amplitude and frequency relationships of the desired RX signal and the TX leakage signal present after RX down conversion. Even though the TX leakage signal is shifted in frequency from the desired RX signal, a TX IM2 signal is present in the same range of frequencies as the RX signal. The TX IM2 signal results from a second order non-linearity in the receive path of the transceiver.
Therefore, it would be desirable to develop a transceiver architecture absent SAW filters that is not adversely affected by the above described inter-modulation interference.